This invention relates to optical devices and related methods of manufacture and verification. In particular, but not exclusively, the invention relates to security anti-counterfeit devices employing the principle of optical diffraction, and to an improved form of optical security device for use in the protection of documents and articles of value from counterfeit and to verify authenticity.
Several forms of such devices are known and used to prove the authenticity of items of value and to prevent their fraudulent duplication. Examples of such items are banknotes, plastic cards, value documents such as fiscal stamps, travel documents such as passports and also valuable goods per se.
Devices based on the principle of optical diffraction have found use for such purposes since they can produce, by the process of optical diffraction, an optically variable image with characteristic features such as depth and parallax (holograms) and movement features and image switches (purely diffraction grating devices and some holographic devices). Such diffractive, optically variable image forming devices are used as anti-counterfeit devices both because their effects are highly recognizable and cannot be duplicated by print technologies, and because specific, and difficult-to-replicate, optical and engineering techniques are required for their production.
Such diffractive, optically variable image-forming devices are generally manufactured to produce effects based on holographic or pure diffraction grating techniques and often comprise embossed surface relief structures as those known disclosed in Practical Holography, Graham Saxby, Prentice Hall 1988. These device structures are typically applied to documents of value, plastic cards and articles of value to be protected in the form of holographic or diffractive hot stamping foil or holographic or diffractive labeling, often tamper evident.
There are various forms of pure diffraction grating devices already known and in use as such security devices. One example is disclosed in U.S. Pat. No. 4,568,141, which describes a diffraction optical authenticating element that provides a color pattern moving at a predetermined velocity along a predetermined track when the document is illuminated from a first direction, and viewed from a second direction. The device described consists of a plane diffraction grating structure which defines a predetermined track in which at least one of the spatial frequency, or angular orientation, varies along said track such that when the device is illuminated and rotated in a plane, adjacent regions of the diffraction grating structure successively diffract light to cause a viewer to see a color pattern which appears to move along the movement track. Each element of this device comprises a pure plane diffraction grating and does not form an image outside of the plane of the device.
A manufacturing method for such a security diffraction grating master is disclosed in U.S. Pat. No. 4,761,252 and employs a technique using a punch to impress successive small areas of a flexible embossing die into a sheet of thermoplastic material.
Further, U.S. Pat. No. 5,034,003 discloses another form of optical security device using diffraction gratings to produce a switching image by recording in the device sets of pixels with each pixel consisting of small areas of different grating spatial frequencies and orientations and serving to form a diffracted image visible from different directions. In this device a diffractive device switches between two replayed images created by splitting the device into several sets of interlocking pixels, with each pixel divided into sub pixels of plane diffraction gratings with different sub pixels corresponding to different replay directions. This device only forms images located on the image plane of the device. There is no provision for additional functionality within the image such as enhanced view angle or depth effects to provide parallax effects.
Another known method of producing a pure diffraction grating security device is to write directly the diffractive structure by use of electron beam lithography, such as those known in the art as Catpix, Pixelgram, and Exelgram. Some examples of this are WO-A-9103747, WO-A-9428444, WO-A-9318419, WO-A-9504948 and WO-A-9502200, WO-A-9823979 and WO-A-002067 describing electron beam generated diffractive optical security devices.
WO-A-9103737 describes a method of subdividing an optically invariant image into a set of pixels which are mapped to diffraction spatial frequencies where a greyness factor for each pixel determines the extent of curvature of the grating lines in each pixel. WO-A-9318419 describes a pixellated optical diffraction device in which each pixel is an individual optical diffraction grating where the pixels are arranged in groups containing a multiplicity of pixels according to a predetermined rule, typically mapping to larger pixels of an optically invariant image, in order to produce a visually observable optically variable effect. WO-A-9428444 describes a pixellated diffractive device including a multiplicity of pixels in turn divided into multiple sub pixel arranged in gridded arrays, where the sub pixels are arranged in groups to cooperate to generate a multiple image diffractive device upon illumination. It should be noted that all of the above techniques are limited to subdividing images into rectangular pixels that generally contain diffraction gratings of various types. WO-A-950498 details the structure of a surface relief diffractive device which generates multiple diffractive image components under illumination where the diffractive structure is divided into multiple discrete tracks. WO-A-9823979 describes the creation of a color diffractive device by again splitting an image into a gridded rectangular pixel array and further sub-dividing this array into component diffraction gratings whose relative areas are adjusted to control hue and intensity of color. WO-A-9917941 describes a pixellated diffusing device in which the diffuse elements are arranged in pixels and further arranged to have grey scale regions created by using structures of different scattering properties. A disadvantage of the ‘Pixelgram’ image pixel arrangements known from the above documents are the discontinuities that are evident between adjacent pixels, and in the Exelgram, the discontinuities between tracks. These inter-element gaps lead disadvantageously to diffuse scatter effects and extraneous diffraction effects. Additionally small pixels or tracks tend to increase scatter and reduce area fill efficiency and brightness. WO-A-002067 describes a diffractive device consisting of background diffractive elements and interstitial diffractive elements arranged such that the diffraction effect of the background elements is modulated by the interstitial elements. The teaching of all of the above documents is incorporated herein by reference.
Diffractive optical variable image forming devices are also known and have been produced by holographic methods, such devices are known for their use in security applications for example on credit cards, banknotes, etc. Examples of teachings on such holographically manufactured security structures can be found in U.S. Pat. Nos. 5,694,229, 5,483,363 and WO-A-9959036. The optical recording and manufacturing methods and other teachings of these documents are incorporated by reference. Again these teachings are limited particularly in their flexibility and range of covert and microscopic features that maybe incorporated.
Some teachings also exist in relation to machine readable, or coherently viewable, holographic or diffractive structures as used for security. For example U.S. Pat. No. 4,544,266 reveals the authentication of a document by the use of a machine readable diffraction based encoded mark that is difficult to copy, and U.S. Pat. No. 5,101,184 describes another way of machine reading a diffractive security device by detecting the different intensities of diffracted light produced in different directions by asymmetric relief structures.
Another security device consists of a volume hologram and contains a visually viewable hologram combined with a superimposed laser transmission hologram as disclosed in DE-A-3840037, where the laser transmission hologram image is designed to be non-discernable under white light but designed to be read under coherent laser light using a visualiser or machine reader device.
U.S. Pat. No. 5,483,363 describes how an embossed surface relief hologram can contain a superimposed second diffractive structure designed to be machine readable by creating an out-of-plane image.
Another holographic system and method for determining which of a plurality of visually indistinguishable objects have been marked with a covert indicator is disclosed in U.S. Pat. No. 5,825,475 which reveals a number of usually indistinguishable objects some of which have been marked with a covert holographic indicator which is exposed to be viewed but which is detectable only when illuminated with a coherent reference light of predetermined wavelength designed to be evaluated by a specific form of scanner evaluation unit.